Update Auth.php

Fixed an issue when we try to install baikal on Synology NAS.
touch() function is not supported on this hardaware.

use file_put_contents () does not interfere with the logic of the code and provides better compatibility as touch() also has other problems on Windows OS

http://synology.com

http://blog.idleman.fr/installez-votre-synchronisateurs-de-contacts-chez-vous-avec-baikal/
Merge pull request #142 from torzak/master

Update Auth.php - Thanks @torzak !